Crude oils and other heavier petroleum fraction often contain unwanted fouling deposits within them that may obstruct the flow of hydrocarbon production fluids. These deposits may also occur in the near-wellbore region of the formation, well production tubing, valves and chokes, flowlines, risers, surface treating vessels, and storage tanks. Tank bottoms are often a combination of paraffin, asphaltene, emulsion, and/or inorganic scale.
Conventionally, removal of wax, paraffins, asphaltenes, and the like has been achieved by mechanical, chemical or thermal methods. However, the mechanical and chemical methods can be costly and time consuming. Moreover, the chemical methods use additives that are harmful to the environment. Thermal treatment, may be performed with bottom hole heaters, hot oil, hot water or steam.
Various chemical methods for removing these types of deposits have been utilized in the past. Other solvents and dispersants such as a copolymer of a primary alcohol and ethylene oxide with sodium silicate and N-substituted succinimide ethers have been tried. A mixture of an alkyl or aralkyl polyoxyalkylene phosphate ester surfactant in free acid form or as a salt with a mutual solvent and water may be injected to remove paraffin deposits. This mixture must be at a temperature greater than the melting point of the wax to be effective. Since none of these processes melt the wax, they can only slowly eat away at its surface. This is not fast enough at most realistic surface-to-volume ratios. Furthermore, they create dispersions in water which must be disposed of or otherwise expensively dealt with.
The use of inorganic nitrate/nitrite compounds in redox reactions may result in an exotherm that melts the paraffin deposit and generates nitrogen gas. This technique does melt the wax, but requires the use of water to deliver the reactants, so that if the wax disperses at all, which it may well not, it does so into water which then must be expensively dealt with. Furthermore, gas generating redox reactions tend to be self accelerating, rendering them at best kinetically unpredictable, and at worst explosive.
An acid compound and a neutralizer compound may react exothermically to melt the paraffin wax deposits and form a dispersant to remove the melted fragments of the deposit from the surfaces of hydrocarbon (oil and/or gas) production equipment. Examples of acids used in this method include H3PO4, H2SO4, and HCl, whereas examples of neutralizers used include NaOH, KOH, MgO, MgCO3 and NaHCO3.
It would thus be desirable if other methods were devised to generate in situ heat downhole with heat-generating chemicals that are easier to handle and allow for a more efficient removal of fouling deposits from the near-wellbore region. It would also be desirable for the heat-generating chemicals to be predictable in terms of placement and functionality.